Polyester films of various compositions are known to the art. These films, which may be continuously extruded into sheets of various thicknesses, have good tensile strength and modulus, and have found use, among other things, as magnetic media substrates.
To date, much attention in the art has been focused on the optical properties of multilayer films. Alfrey et al., Polymer Engineering and Science, Vol. 9, No. 6, pp. 400-404 (November 1969), Radford et al., Polymer Engineering and Science, Vol. 13, No. 3, pp. 216-221 (May 1973), and U.S. Pat. No. 3,610,729 (Rogers), for example, describe the reflectivity of certain multilayer polymeric films. This work has been extended to multilayer polyester films. Thus, U.S. Pat. No. 3,801,429 (Schrenk et al.) and U.S. Pat. No. 3,565,985 (Schrenk et al.) disclose multilayer composites made from various resins, including polyesters, and methods for making the same. The composites have the property of being iridescent, even without the addition of pigments.
U.S. Pat. No. 4,310,584 (Cooper et al.) describe the use of polyesters in making multilayer iridescent light-reflecting film. The film includes alternating layers of a high refractive index polymer and a polymer with a low refractive index. The high refractive index polymer is a cast nonoriented film that includes a thermoplastic polyester or copolyester such as polyethylene terephthalate (PET), polybutylene terephthalate and various thermoplastic copolyesters which are synthesized using more than one glycol and/or more than one dibasic acid.
U.S. Pat. No. 5,122,905 (Wheatley) describes a multilayer reflective film with first and second diverse polymeric materials in alternating layers that exhibits at least 30% reflection of incident light. The individual layers have an optical thickness of at least 0.45 micrometers, and adjacent layers have a refractive index difference of at least 0.03. U.S. Pat. No. 5,122,906 (Wheatley) describes similar reflecting bodies, wherein a substantial majority of individual layers have an optical thickness of not more than 0.09 micrometers or not less than 0.45 micrometers, and adjacent layers have a refractive index of at least 0.03.
Some attempts have also been made to improve the mechanical properties of particular multilayer films. Thus, U.S. Pat. No. 5,077,121 (Harrison et al.) describes polyethylene-based multilayer films consisting of layers of two or more different resins, wherein the draw ratios of the composite film are found to exceed the draw ratios of monolithic films of the component materials. In the films described, a layer of high elongation, low modulus material is sandwiched between layers of low elongation, low modulus material. The reference also notes that a similar phenomenon is sometimes observed in composites wherein a high modulus, low elongation material is sandwiched between layers of high elongation material, although in many of these composites, the low elongation material fails at its characteristic low elongation, causing a simultaneous, premature failure of the high elongation layers.
To date, however, relatively few improvements have been made in the mechanical properties of multilayer polyester films, despite the fact that such films have become increasingly important in a wide variety of commercial applications. While polyester films are already available which have a high modulus and medium elongation, in a variety of uses, as when polyester films are used as engineering materials or are subject to winding operations, the physical limitations of these films are already being tested. There thus remains a need in the art for a multilayer polyester film having improved mechanical properties, and for a method of making the same. In particular, there is a need in the art for multilayer polyester films having improved tensile modulus, tensile strength, and stretchability.
A further problem encountered with polyester films, and frequently commented on in the literature, relates to the incidence of hazing. Hazing in polyester films is undesirable in applications where a clear film would be preferred, as in window films. In other applications, a particular degree of hazing is acceptable or even desirable. To date, however, the phenomenon of hazing has been poorly understood, and no methods have been provided which allow for easy control of the degree of hazing in polyester films. There is thus a need in the art for a method of controlling the degree of hazing in polyester films, and particularly in multilayer polyester films. In particular, there is a need in the art for a method of producing multilayer polyester films with any desired degree of hazing, through manipulation of readily controllable process parameters.
Yet another problem encountered in polyester films relates to their coefficient of friction. Thin polyester films having a high coefficient of friction are prone to wrinkling, web breaks, and similar damage during winding and handling. In these applications, it would be desirable to use a polyester film having a lower coefficient of friction, so that adjacent surfaces of the film would slide over each other easily.
To date, this has been accomplished through the use of slip agents. However, the use of slip agents is undesirable in that it complicates the manufacturing process, and frequently compromises the mechanical or optical properties of the resulting film. There is thus a need in the art for polyester films which are substantially devoid of slip agents, but which have a comparatively low coefficient of friction. There is also a need in the art for a method of controlling the coefficient of friction in a polyester film without the addition of slip agents.
These and other needs are met by the present invention, as hereinafter disclosed.